Device for forming an autostereoscopic image

ABSTRACT

A device for forming autostereoscopic images by implementing a cylindrical lens array, the device being characterized in that it comprises in succession: 
     an entrance objective (L1, L2); 
     a lens array (20) having diverging elementary cylindrical lenses disposed substantially in the image focal plane of the entrance objective, said array having a focal length such that for an image area equal to the pitch (p) of the lenses making it up, the image of the entrance pupil of the entrance objective (L1, L2) has a nominal width equal to said pitch; and 
     a converging transfer optical system (L3, L4) for forming an orthostereoscopic real image.

The present invention relates to a device for forming autostereoscopicimages by using an array of cylindrical lenses.

A device for forming autostereoscopic and orthostereoscopic images isknown from European patent application EP-84598, which device comprisesboth a first lens array having convex lenses that form an image which isautostereoscopic and pseudostereoscopic (i.e. in "inverted relief"), anda second lens array having convex lenses which form an orthostereoscopicimage.

That device suffers from the drawbacks of requiring two lens arrays,thereby giving rise to very severe constraints concerning tolerances andalignment.

European patent EP 305 274 in the name of the Applicant describes amethod whereby elementary images are individually reversed about theirown axes or centers of symmetry by means of an electronic process, andare recombined so as to supply an orthostereoscopic image.

The methods of the prior art for obtaining an orthostereoscopic imagethus require the implementation of a system that has two stages.

The problem posed by the invention is to obtain directly an image whichis autostereoscopic and orthostereoscopic.

To this end, the invention provides a device for formingautostereoscopic images by implementing a cylindrical lens array, thedevice being characterized in that it comprises in succession:

an entrance objective;

a lens array having diverging elementary cylindrical lenses disposedsubstantially in the image focal plane of the entrance objective, saidarray having a focal length such that for an image area equal to thepitch (p) of the lenses making it up, the image of the entrance pupil ofthe entrance objective has a nominal width equal to said pitch; and

a converging transfer optical system for forming an orthostereoscopicreal image.

Downstream from the pupil P₁ of the entrance objective, and inparticular between the objective and the lens array, the device mayinclude a cylindrical element that is crossed relative to the lens arrayand that compensates astigmatism, at least in part. The cylindricalelement may be a converging cylindrical lens having a focal lengthsuitable for compensating said astigmatism, at least in part. In avariant, the cylindrical element may be a second diverging lens arraywhose focal plane coincides substantially with that of the firstdiverging array.

The invention also provides an image-forming device characterized inthat it comprises an image sensor and in that the transfer opticalsystem projects the rays emerging from the lens array onto the imagesensor, the image of the lens array in the transfer optical system beingsuch that the pitch of the lenses of the lens array corresponds thereinto an integer number of image points (pixels) of the image sensor, andthe image of the pupil of the entrance objective is situatedsubstantially at the pupil of the transfer optical system.

The image sensor may be constituted by a charge coupled sensor, inparticular constituted by a set of three individual sensors associatedwith a prismatic three-color beam-splitter forming images on the threesensors that are nominally aligned with one another image point by imagepoint.

The lens array may be oriented in the direction of the lines of theimage sensor. The entrance objective, which is advantageouslytelecentric, may include an entrance lens whose pupil is substantiallyequal to 100 mm. The lens array may have a pitch of 0.4 mm. The transferoptical system may have a magnification substantially equal to 0.1. Itmay have a circular diaphragm, in particular of the iris type.

The invention also provides an autostereoscopic video system includingan image forming device as defined above.

Other characteristics and advantages appear more clearly on reading thefollowing description, given by way of non-limiting example and withreference to the accompanying drawing, in which:

FIG. 1 and FIG. 1a shows an image forming device and system of theinvention; and

FIGS. 2a and 2b show a preferred embodiment of a device of theinvention.

FIG. 2c shows an enlarged portion of the device of the preferredembodiment of FIGS. 2a and 2b.

FIG. 1 shows a camera device of the invention. It comprises thefollowing elements:

1) An entrance objective that is preferably telecentric comprising anentrance lens L₁ and an exit lens L₂ whose focus F₂ in a telecentricsystem coincides with the optical center O₁ of the lens L₁. Such anentrance objective is known per se from European patent applicationEP-A-O 84998 (CNRS). When the optical system is telecentric, the imageof the central point of the entrance pupil of the lens L₁ is projectedto infinity by the lens L₂, thereby achieving parallelism that ensuresthe lens array is approached favorably. In particular, the two lenses L₁and L₂ may be conjugate, i.e. the focus F₁ of the lens L₁ may alsocoincide with the optical center O₂ of the lens L₂. The objective L₁ mayhave a focal length of 200 mm and an aperture of f/2, for example, whichcorresponds to a working diameter for the pupil of 100 mm, whichdistance constitutes the working stereoscopic baseline for takingpictures. This value which is slightly greater than the spacing betweenthe eyes of an observer (or inter-pupil distance, about 65 mm), isparticularly favorable for obtaining realistic stereoscopic perspectiveafter projection onto a screen.

2) A lens array having an area of about 70 mm×90 mm and made up ofelementary lens 10 that are divergent, i.e. concave, which are disposedvertically at a pitch p of 0.4 mm. The lens array is disposedsubstantially at the focus F of the entrance objective and slightlyupstream therefrom (in the light ray propagation direction). Each of theelementary lenses has a focal length such that for an image area equalto the pitch p of a microlens, i.e. of width 0.4 mm, the image of thepupil of the objective F₁ formed through each of the concave elementarylenses is exactly 0.4 mm. This ensures that all the virtual images ofthe pupil formed by each elementary lens (or microlens) touch oneanother exactly. It may be observed that since the array 20 is made upof cylindrical type lenses, the dimensions of the images of the pupilare naturally taken into consideration in the horizontal plane only. Thediverging array 20 may be manufactured by techniques similar to thoseused for making converging arrays, by calendaring a plate ofthermoplastic material.

3) A transfer optical system which is preferably orthoscopic, i.e. whichdoes not induce deformations in vertical lines, and which may comprise afield lens L₃ positioned downstream from the lens array 20 to projectall of the light rays from the array 20 towards an image transferobjective L₄. The objective L₄, e.g. having a focal length of 25 mm, ismounted on a camera 22 that is provided with charge coupled sensors.This transfer optical system L₃, L₄ transfers the virtualorthostereoscopic image formed by the lens array 20 and forms a realimage 21 immediately upstream from the sensors of the camera 22. Themagnification of the transfer optical system L₃, L₄ is selected so thatthe rays emerging from the lens array 20 are applied to the camera 22under conditions such that the image 21 has a pitch p' corresponding toan integer number of image points (pixels) of the image sensor 22. Inaddition, the distance between the image 21 and the image sensor 22 issuch that focusing takes place on the sensor(s) of the camera 22. Thelens array 20 acts in one direction only (horizontal). A linear objecthaving a horizontal axis and placed at infinity gives a virtual image inthe focal plane P of the array 20 that is situated upstream therefrom. Alinear object having a vertical axis and placed at infinity gives a realimage substantially at the focus F of the entrance objective (L₁ , L₂),said focus F necessarily being situated downstream from the divergentlens array 20. This gives rise to astigmatism which, in this case,interferes with focusing.

To compensate the astigmatism, it is possible to place a convergingcylindrical lens 40 of long focal length e.g. downstream from the pupilP₂ of the entrance objective, and preferably between L₁ and L₂, thegenerator lines of the converging lens being horizontal (so that it iscrossed relative to the lens array 20 which is disposed vertically). Thefocal length is designed to move towards each other and preferably tosuperpose the convergence point for vertical objects and the focal planeF of the diverging array.

For horizontal objects at infinity, light rays converge on the focus Fand a virtual image is formed in the plane P. For vertical objects atinfinity, the cylindrical lens 41 crossed with the lens array 20 has theeffect that the real images thereof are formed in the plane P.

For objects placed at a given finite distance, and for exactcompensation, the convergence point is situated upstream from the focalplane F.

Another solution is to place a second diverging lens array 41practically in the same plane as the first, having the same focal lengthas the first, or a focal length calculated so that the two focal planescoincide, and having a pitch that corresponds to one pixel (comparedwith the one-fourth pitch of the first array for square pixels and fourviewpoints). The pupil parameters are then fixed.

In general, the magnification of the transfer optical system is selectedas a function of the size of the real image it is desired to obtain.This real image is orthostereoscopic because the virtual image producedby the diverging elementary lenses 10 does not have the inversion thatis produced by the converging lenses of the prior art.

The elements of the entrance objective and of the transfer opticalsystem are disposed in such a manner that the image of the pupil of theentrance objective coincides substantially with the pupil of thetransfer optical system. When the entrance objective is not telecentricthis condition serves, in particular, to ensure that the transferoptical system restores parallelism as explained below.

In particular, the sensor 22 may comprise three charge coupled sensors24, 25, and 26 as best illustrated in FIG. 1a mounted on a prismaticthree-color beam-splitter 23, which sensors are accurately aligned sothat the first pixel of the first line coincides for all three sensors,and in general so that the images from the three sensors 24, 25, and 26are thus in alignment pixel by pixel.

The signals seen by the sensor 22 integrated in a camera 30 may beapplied to a television monitor 31 equipped to display stereoscopicimages, or they may be transmitted via a transmitter 32 so as to bereceived by a receiver 33.

EXAMPLE

A lens array 20 having a pitch of 0.4 mm and a focal length of 1.66 mmwas placed at 20 mm from the optical center of L₂ and at 90 mm from theoptical center of L₃. The lens L₁ was constituted by a doublet L'₁, L'₂.Its pupil is referenced P₁.

L₁ focal length f₁ =200 mm

L₂ focal length f₂ =300 mm

L₃ focal length f₃ =230 mm

L₄ focal length f₄ =25 mm

distance O₁ O₂ between the optical centers of the lenses L₁ and L₂ O₁ O₂=180 mm

distance O₂ O₃ between the optical centers of the lenses L₂ and L₃ O₂ O₃=110 mm

distance O₃ O₄ between the optical centers of the lenses L₃ and L₄ O₃ O₄=245 mm.

The device is also advantageous for the following reasons.

To take a set of pictures in three dimensions, it is necessary for thesystem to enable a scene to be observed from different viewpoints andfor the number of viewpoints to be greater than or equal to 2, with eachviewpoint being far enough apart from the preceding viewpoint for thereto be significant difference (or disparity) between the views. When thepicture is taken using a single objective, without movement of itscomponent elements in the plane parallel to the image plane, the entirerelative offset of the picture-taking axes must be contained within thehorizontal diameter of the pupil of the objective which constitutes thetotal available stereoscopic baseline. In the example described above,the total stereoscopic baseline, or the working horizontal diameter ofthe pupil, is equal to 100 mm, which is more than the inter-pupildistance of an adult human being (about 65 mm). In order to obtain astereoscopic baseline of 10 cm with an objective that does not sufferfrom significant defects, and in order to ensure that the perspective ofthe filmed scene is not different from that perceived by an observer, ithas been found experimentally that a ratio of about 2 between the focallength and the working horizontal diameter of the pupil gives thelooked-for results. That is why the above-specified example uses anobjective having a lens L₁ with a focal length of 200 mm and an apertureof f/2.

The focal length should not be considered as such since account must betaken of the dimensions of the sensitive area used. For a standardtri-CCD camera provided with sensors forming a target of about 8.8mm×6.6 mm, the focal length defines an object field that is very narrow,indeed smaller than one-tenth of the field (about 160 mm) provided bythe "standard" focal length for such an area (i.e. about 16 mm). Thesolution to this problem, which is how to reconcile an adequatestereoscopic baseline with a standard focal length, is to separate thesetwo incompatible requirements by using a first intermediate image planeof greater area, e.g. ten times greater. This area is physicallyembodied by the lens array having a working area of 80 mm×60 mm. Thisimage is transferred by a second objective of short focal length, e.g.25 mm, mounted on the camera so as to cause the image formed of thearray to coincide with the charge coupled sensors CCD. Once thestereoscopic baseline has performed its function in forming the image onthe array of vertical cylindrical lenses, it is possible to reduce theimage by transferring it in air while conserving the angle of the objectfield.

More particularly, implementing both the objective L₁ L₂ (which ispreferably telecentric) and the transfer device L₃, L₄ makes it possibleto reduce dimensions by about 10 in the above example, given that theworking area of the first image plane is about 60 mm×80 mm. Since thelens array 20 is placed substantially at the first image plane of theoptical system L₁, L₂, this makes it possible to retain the benefit ofthe 10 cm stereoscopic baseline in spite of the image format beingreduced on the sensor 22. The use of an initial area of 60 mm×80 mmmakes it possible simultaneously to combine the field which is a littlegreater than the standard focal length for this format (160 mm) and thelong stereoscopic baseline which is equal to 10 cm. The signal outputfrom the sensor 22 is directly usable because the image formed on thesensor is orthostereoscopic.

The picture taking device of the invention makes it possible firstly touse only one array 20 for all three colors and secondly for this arrayto be of large dimensions, thereby making it easier to manufacture andposition with the desired accuracy. This avoids the drawbacks both ofFIG. 1 (array of small dimensions that is difficult to position in thesensor, which in any case does not avoid the geometrical distortionsintrinsic to that geometry), and of FIG. 2 (large number of lens arrayswhich are practically impossible to keep in alignment except under verysevere experimental conditions).

In a preferred embodiment corresponding to a correction implementing acylindrical lens 40, the second objective L₃, L₄ for transferring theimage has an iris diaphragm. Such a diaphragm is equivalent to adiaphragm in the form of a horizontal slot in the first objective L₁,L₂, but it is easier to position since the only parameter is itscentering. The centered iris diaphragm of the second objective isequivalent to a diaphragm in the form of a horizontal slot in the firstobjective. Light rays coming from the first pupil are not disturbed inthe direction parallel to the axis of the microlenses, whereas in thehorizontal direction these rays are definitively associated with theimages of the pupil as obtained by each microlens. The images of thepupil cannot be affected by the reduction in the size of the pupil inthe second objective. When the correction is performed by the secondlens array 41 that is crossed with the first, the second diaphragm iseffective only concerning the amount of light received, the entrancepupil being fully determined by the two arrays.

Because of the discrete nature of the sensors 24, 25, and 26 of thecamera it is possible to avoid subdividing the pupil into as manysub-pupils as there are selected viewpoints. During image transfer, theimage of the array 20 is positioned so that each image of each lens (ormicroimage of the pupil) is formed on an integer number of image points(or pixels) equal to the number of viewpoints. Because of thereversibility of light paths, the discrete nature of the sensitivesurface of CCD sensors causes the first pupil of the system to bediscrete in nature. The fact that the microimages of pupil No. 1 asformed at the lens array (in the manner of a continuum) are projected ona structure that is discrete both from the space point of view and alsofrom the energy point of view makes it possible to subdivide the pupilinto as many distinct geographical zones equal in number and in relativedisposition to the pixels put into exact correspondence with the lensesof the array. In the above example, each image of a microlens is formedhorizontally on four pixels, thereby splitting up the main pupil intofour equal zones separated by portions that are made blind because theycorrespond to the interpixel gaps of the CCD sensors. The horizontalstructure selected for the sensitive surface determines the resultingstructure of the working pupil used for taking pictures in relief, andconsequently determines the means for processing the resulting image.The fact of using four pixels per microlens leads to four viewpointsbeing filmed simultaneously (one viewpoint per sub-pupil). Electronicprocessing of the image becomes possible because the processing isperformed on the smallest entity of the resulting composite image: thepixel, thus achieving excellent separation between the viewpoints.

Even better stereoscopic separation can be obtained by placing the linesof the sensor 22 in a direction parallel to the axis of the lenses ofthe lens array 20. The separation between adjacent image pointsbelonging to different lines is greater than that between adjacent imagepoints belonging to the same line. This corresponds to positioning at90° relative to the usual conditions (vertical line scanning) but usualconditions can be reestablished, if so desired, by appropriateelectronic processing.

I claim:
 1. A device for forming autostereoscopic images by implementinga cylindrical lens array, the device comprising in succession:atelecentric entrance objective having an image focal plane and anentrance pupil; a lens array having diverging elementary cylindricallenses disposed substantially in the image focal plane of the entranceobjective, said lenses having a longitudinal axis, and said array havinga focal length such that the image of the entrance pupil of the entranceobjective has a width equal to a pitch of said lenses; and a field lensto project substantially all light rays from said array towards aconverging transfer objective, said transfer objective forming anorthostereoscopic real image from said light rays.
 2. A device accordingto claim 1, including, downstream from the entrance objective, acylindrical element that is crossed relative to the lens array and thatat least partly compensates for astigmatism.
 3. A device according toclaim 2, wherein the cylindrical element is a converging cylindricallens having a focal length for at least partly compensating saidastigmatism.
 4. A device according to claim 2, wherein the cylindricalelement is a second diverging lens array whose focal plane substantiallycoincides with that of the first diverging lens array.
 5. A deviceaccording to claim 1 comprising a charge-coupled video image sensor forgenerating video images and in which the transfer optical systemprojects the rays emerging from the lens array onto the image sensor,the image of the lens array in the transfer objective being such thatthe pitch of the lenses of the lens array is equal to an integer numberof image points of the image sensor, and the image of the pupil of theentrance objective is situated substantially at the pupil of thetransfer objective.
 6. A device according to claim 5 wherein thecharge-coupled video image sensor comprises a set of three individualsensors associated with a prismatic three-color beam-splitter formingimages on the three sensors that are in mutual alignment, image point byimage point.
 7. A device according to claim 5, wherein said image sensorhas a line direction that is parallel to the lines of said video imagesand wherein the lens array is oriented in said line direction of saidimage sensor.
 8. A device according to claim 1, wherein the entranceobjective comprises an entrance lens whose pupil is substantially equalto 100 mm.
 9. A device according to claim 1, wherein the transferobjective has a magnification substantially equal to 0.1.
 10. A deviceaccording to claim 1, wherein the transfer objective has a circulardiaphragm.
 11. A device according to claim 1, wherein the convergingtransfer objective is orthoscopic.
 12. An autostereoscopic video systemincluding an image-forming device according to claim 1.